As a transmission for use in automobiles, a metal belt-type, chain-type or toroidal continuously-variable transmission has been developed. The continuously-variable transmission carries out power transmission by the friction coefficient between the chain and the pulley therein, and therefore, the lubricating oil for continuously-variable transmission to be used for these is required to have an intermetallic friction coefficient above a certain level.
On the other hand, as a startup device, a torque converter is mounted. The torque converter transmits power while absorbing differential rotation by stirring of a lubricating oil for continuously-variable transmission, but not at a time of startup, it transmits power via a rock-up clutch to thereby reduce energy loss to secure fuel saving. For rock-up clutch control, slip control to transmit power with slipping is carried out in addition to direct fastening, and when the friction characteristics of the lubricating oil for continuously-variable transmission are unsuitable, self-excited vibration called shudder occurs. Consequently, the lubricating oil for continuously-variable transmission is required to have shudder-resistant performance. By improving the shudder-resistant performance of a lubricating oil for continuously-variable transmission, more accurate rock-up clutch control becomes possible, thereby contributing toward fuel saving by automobiles.
As a continuously-variable transmission oil, PTL 1 discloses a lubricating oil composition containing a lubricating base oil, an alkaline earth metal sulfonate or phenate, an imide compound and a phosphorus compound and satisfying a specific metal friction coefficient and a shudder preventive lifetime.
PTL 2 discloses a lubricating oil composition prepared by blending at least one phosphorus-containing compound selected from a phosphate monoester, a phosphate diester and a phosphite monoester each including a hydrocarbon group having 1 to 8 carbon atoms, and a tertiary amine compound having, as a substituent, a hydrocarbon group having 6 to 10 carbon atoms, in a base oil formed of a mineral oil and/or a synthetic oil.
PTL 3 discloses a lubricating oil composition prepared by blending a tertiary amine having a specific structure, at least any one of an acidic phosphate and an acidic phosphite, and at least any one of a metal sulfonate, a metal phenate and a metal salicylate, in a lubricating base oil.
PTL 4 discloses a lubricating oil composition prepared by blending at least any one of a primary amine, a tertiary amine, a metal sulfonate, a metal phenate and a metal salicylate, and at least any one of an acidic phosphate and an acidic phosphite, in a lubricating base oil.
The original purpose of a lubricating oil is to protect a friction surface by an oil film formed in the sliding part thereof. Accordingly, for forming a strong oil film, it is advantageous that the lubricating oil has a high viscosity. However, in the case of using a lubricating oil having a high viscosity, the power necessary for stirring the lubricating oil and for oil filling is a large energy loss to interfere with energy saving and fuel saving performance. Consequently, it is intended to lower the viscosity of a lubricating oil for reducing energy loss, in recent years.